This invention relates to a system for amplifying laser beams to very high power levels using a segmented amplifier in which each segment includes a suitably excited laser medium and a reflector which totally or partially reflects the amplified laser beam.
There are many problems associated with the reliable operation of powerful laser systems. These arise from such defects as poor excitation efficiency of the laser medium, quality of the various optical components inserted into the path of the laser beam, e.g., beam expanders, electro- and magneto-optic switches, quality of the laser beam and the control of the environmental parameters under which the laser system is operated.
The trend to high laser output power intensifies the present problems associated with larger diameter, higher quality and intense beams. The present invention provides for a relatively compact, ultra high power laser amplifier system which can produce very large diameter, high quality laser beams using very few optical components.
The power output of a laser amplifier system can be approximated by the relationship EQU P = (e A/t) watts (1)
where e is the safe loading of the laser medium, or its container in joules cm..sup..sup.-2, A the effective area of the output aperture in cm..sup.2 and t the duration of the laser output pulse. For continuous wave output t will be taken to be 1 second.
Laser applications for example in the fields of controlled thermonuclear fusion and non-linear optics demand extremely high peak power output from laser systems. The situation has now been reached where the required increases in the peak power output P relies to a greater extent on increasing the output area A because it is becoming increasingly difficult to improve the radiation loading e and to shorten the pulse duration t.
This invention provides a solution to the problem of generating very high laser output powers by allowing for large output areas A, but at the same time minimizing the most detrimental effects of large radiation loadings e in conjunction with both large and small values of the pulse duration t in a relatively compact amplifier arrangement.
A major difficulty with high power laser systems is to minimise the destructive effects, e.g., self-focussing or beam collapse of large e/t ratios. For example, in present high power rod-disc neodymium doped glass laser systems e values lie in the range 0.1 to 1 joules cm..sup..sup.-2 for t values ranging from 10.sup..sup.-11 to 10.sup..sup.-10 seconds, i.e., e/t values of between 10.sup.9 and 10.sup.10 watts cm..sup..sup.-2 as a relatively safe operating range.
If the possibility of improving the e/t ratio is neglected then the way to higher powers must involve the use of multiple laser beams or a very large single beam. Current high power neodymium doped glass laser systems being developed for laser fusion studies use both these techniques in that they have as many as twelve laser beams, each one of large diameter up to 30 cm., producing between 10.sup.12 and 10.sup.13 watts peak power per system.
Unfortunately, the area of the output aperture cannot be increased arbitarily because large output aperture areas A lead to factors which can drastically affect the quality of the laser beam, and hence the destructiveness associated with a given e/t ratio.
The present invention allows for a very high quality laser beam by relying on the natural divergence of the beam to provide large diameter outputs and a large separation between segments of laser medium to monitor and maintain the required beam quality. The large segment separator is also used to increase the self-oscillation threshold along the beam amplification path. Uniform excitation of the laser medium is allowed for in the present invention. The quality of the laser media determines the ultimate effectiveness of the present invention and improvements in the laser media will have a corresponding improvement in their effectiveness.
Prior art high power systems utilize laser medium segments in a straight line extending over lengths of several tens of meters. The expansion of the beam is usually accommodated by the insertion of collimators, i.e., beam-expanding telescopes into the path of the laser beam. The self-oscillation of such a system is prevented by the insertion of electro- and magneto-optic switches. One disadvantage is that to achieve ultra high power beam outputs the length of the chain would have to be up to one kilometer or longer. Another disadvantage is that the inclusion of the collimators and switches causes a deterioration in beam quality, which in turn leads to damage in the laser media due to non-uniformity of the laser beam.